Method and apparatus requiring fewer number of look-up tables for converting luminance-chrominance color space signals to RGB color space signals

ABSTRACT

In a method and apparatus for converting digitized luminance-chrominance color space signals to digitized RGB color space signals, a first combining unit generates a plurality of predetermined linear combinations of the chrominance color space signals received by a multiplexed multiplication unit which includes no more than two look-up tables that contain digitized transformation values for performing matrix multiplications of the linear combinations of the chrominance color space signals. A second combining unit linearly combines the digitized transformation values outputted by the multiplexed multiplication unit and a predetermined binary combination of the luminance color space signal in a first predetermined manner to generate three RGB color combination signals. A third combining unit linearly combines the RGB color combination signals in a second predetermined manner to obtain the RGB color space signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for performing colorspace conversion, more particularly to a method and apparatus requiringa fewer number of look-up tables for converting digitizedluminance-chrominance color space signals to digitized RGB color spacesignals.

2. Description of the Related Art

It is desirable to merge a video signal with graphic signals in amulti-media computer system. The video signal may come from a televisionimage processing system having a capture or frame grabbing capability,or from a compressed video playback of a CD-ROM or network transmission.Color space conversion is needed in image processing applications toconvert luminance-chrominance color space signals, which offer theadvantages of a lower transmission bandwidth and a lower data storagerequirement, into RGB color space signals, which are used whendisplaying an image on a computer monitor.

CCIR 601, which was proposed by the Comite Consultatif International desRadiocommunications (CCIR), establishes the following formulas forconverting from the YCbCr luminance-chrominance color space to the RGBcolor space:

    R=Y+1.402(Cr-128)                                          (a.1)

    G=Y-0.714(Cr-128)-0.344(Cb-128)                            (a.2)

    B=Y+1.772(Cb-128)                                          (a.3)

If U and V are used to represent the shifted chrominance components(Cb-128) and (Cr-128), respectively, Equations (a.1) to (a.3) can berewritten as follows:

    R=Y+1.402V                                                 (b.1)

    G=Y-0.714V-0.344U                                          (b.2)

    B=Y+1.772U                                                 (b.3)

where Y ranges between 0, 255!, and U and V range between -128, 127! inan 8-bit representation for each of the Y, Cb and Cr color spacecomponents.

Color space conversion is often implemented by employing multipliers orlook-up tables to achieve the matrix multiplication operations. Look-uptables are preferred because of their less complicated constructions. Itis noted that the matrix multiplication operations dominate the hardwarecomplexity of a color space converting apparatus. As such, the number oflook-up tables is critical in determining the cost of implementing thecolor space converting apparatus. To implement the YCbCr to RGB colorspace conversion of Equations (a.1) to (a.3), a conventional color spaceconverter usually requires four look-up tables to perform the matrixmultiplication of chrominance components. Although the use of fourlook-up tables is less expensive to implement as compared to anotherconventional color space converter which uses a 3-by-3 multiplicationmatrix, a further reduction in the number of look-up tables isdesirable.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a methodand apparatus requiring less than four look-up tables for convertingdigitized luminance-chrominance color space signals to digitized RGBcolor space signals.

Accordingly, it is found that Equations (b.1) to (b.3) can be rearrangedas follows to result in RGB color combination signals by linearlycombining the conversion formulas:

    R-G=0.714(2V)+0.344(U+2V)                                  (c.1)

    B-G=0.714(2U+V)+0.344(2U)                                  (c.2)

    R+B-G=Y+0.714(2U+2V)+0.344(2U+2V)                          (c.3)

    B+G=2Y+0.714(2U-V)                                         (c.4)

    R+G=2Y+0.344(2V-U)                                         (c.5)

Equations (c.1) to (c.5) list a set of possible linear combinations ofEquations (b.1) to (b.3). Note that Equations (c.1) to (c.5) use onlytwo coefficients, namely 0.714 and 0.344, for matrix multiplications.Consequently, no more than two look-up tables may be used to convertluminance-chrominance color space signals to RGB color combinationsignals. Therefore, conversion from the luminance-chrominance colorspace to the RGB color space can be implemented using fewer than fourlook-up tables by converting the luminance-chrominance color spacesignals to the RGB color combination signals expressed as a function ofpredetermined linear combinations of the chrominance color space signalsas defined by the appropriate conversion formulas, and by linearlycombining the resulting RGB color combination signals to obtain the RGBcolor space signals.

According to one aspect of the invention, a method for convertingdigitized luminance-chrominance color space signals to digitized RGBcolor space signals comprises the steps of:

generating a plurality of predetermined linear combinations of thechrominance color space signals and at least one predetermined binarycombination of the luminance color space signal;

providing a multiplexed multiplication unit which receives the linearcombinations of the chrominance color space signals, the multiplexedmultiplication unit including no more than two look-up tables whichcontain digitized transformation values for performing matrixmultiplications of the linear combinations of the chrominance colorspace signals;

linearly combining the digitized transformation values outputted by themultiplexed multiplication unit and the binary combination of theluminance color space signal in a first predetermined manner to generatethree RGB color combination signals; and

linearly combining the RGB color combination signals in a secondpredetermined manner to obtain the RGB color space signals.

According to another aspect of the invention, an apparatus forconverting digitized luminance-chrominance color space signals todigitized RGB color space signals comprises:

a first combining unit for generating a plurality of predeterminedlinear combinations of the chrominance color space signals and at leastone predetermined binary combination of the luminance color spacesignal;

a multiplexed multiplication unit connected to the first combining unitto receive the linear combinations of the chrominance color spacesignals therefrom, the multiplexed multiplication unit including no morethan two look-up tables which contain digitized transformation valuesfor performing matrix multiplications of the linear combinations of thechrominance color space signals;

a second combining unit connected to the multiplexed multiplication unitand the first combining unit, the second combining unit linearlycombining the digitized transformation values outputted by themultiplexed multiplication unit and the binary combination of theluminance color space signal in a first predetermined manner to generatethree RGB color combination signals; and

a third combining unit connected to the second combing unit, the thirdcombining unit linearly combining the RGB color combination signals in asecond predetermined manner to obtain the RGB color space signals.

In one embodiment, the multiplexed multiplication unit comprises firstand second multiplexed look-up tables.

The first multiplexed look-up table includes: a first multiplexer havinga plurality of data inputs which receive selected ones of the linearcombinations of the chrominance color space signals, and a data output,the first multiplexer selecting each of the data inputs thereof insequential first timing phases and providing data present at theselected one of the data inputs to the data output thereof; a first oneof the look-up tables having an input connected to the data output ofthe first multiplexer, the first one of the look-up tables outputtingthe digitized transformation value corresponding to product of the dataat the input thereof and a predetermined first coefficient; and aplurality of first data latches, each of which is connected to the firstone of the look-up tables so as to latch outputs of the first one of thelook-up tables during the sequential first timing phases, respectively.

The second multiplexed look-up table includes: a second multiplexerhaving a plurality of data inputs which receive selected ones of thelinear combinations of the chrominance color space signals, and a dataoutput, the second multiplexer selecting each of the data inputs thereofin sequential second timing phases and providing data present at theselected one of the data inputs to the data output thereof; a second oneof the look-up tables having an input connected to the data output ofthe second multiplexer, the second one of the look-up tables outputtingthe digitized transformation value corresponding to product of the dataat the input thereof and a predetermined second coefficient; and aplurality of second data latches, each of which is connected to thesecond one of the look-up tables so as to latch outputs of the secondone of the look-up tables during the sequential second timing phases,respectively.

In another embodiment, the multiplexed multiplication unit includes nomore than one look-up table and comprises a multiplexed look-up tableincluding: a multiplexer having a plurality of data inputs which receivethe linear combinations of the chrominance color space signals, and adata output, the multiplexer selecting each of the data inputs thereofin sequential timing phases and providing data present at the selectedone of the data inputs to the data output thereof; the look-up tablehaving an input connected to the data output of the multiplexer, thelook-up table outputting the digitized transformation valuecorresponding to product of the data at the input thereof and apredetermined coefficient; and a plurality of data latches, each ofwhich is connected to the look-up table so as to latch outputs of thelook-up table during the sequential timing phases, respectively.

Preferably, a compensating unit is connected to the third combining unitand adds error compensating codes to the RGB color space signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent in the following detailed description of the preferredembodiments with reference to the accompanying drawings, of which:

FIG. 1 is a functional block diagram of the first preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 2 is a functional block diagram of a multiplexed multiplicationunit of the first preferred embodiment;

FIG. 3 is a functional block diagram of the second preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 4 is a functional block diagram of the third preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 5 is a functional block diagram of the fourth preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 6 is a functional block diagram of the fifth preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 7 is a functional block diagram of the sixth preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 8 is a functional block diagram of the seventh preferred embodimentof a color space converting apparatus according to the presentinvention;

FIG. 9 is a functional block diagram of the eighth preferred embodimentof a color space converting apparatus according to the presentinvention; and

FIG. 10 is a functional block diagram of the ninth preferred embodimentof a color space converting apparatus according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the present invention is described in greater detail, it shouldbe noted that like elements are denoted by the same reference numeralsthroughout the disclosure.

Referring to FIG. 1, the first preferred embodiment of a color spaceconverting apparatus for converting digitized YCbCr color space signalsto digitized RGB color space signals in accordance with this inventionis shown to comprise a shifting unit 1, a first combining unit 2, amultiplexed multiplication unit 3, a second combining unit 4, and athird combining unit 5. In this embodiment, color space conversion isperformed according to the following equations:

    R-G=0.714(2V)+0.344(U+2V)                                  (c.1)

    B-G=0.714(2U+V)+0.344(2U)                                  (c.2)

    R+B-G=Y+0.714(2U+2V)+0.344(2U+2V)                          (c.3)

It is noted that Equations (c.1), (c.2) and (c.3) involve threemultiplication operations using the coefficient 0.714, and threemultiplication operations using the coefficient 0.344.

The shifting unit 1 receives the digitized Cb and Cr chrominance signalsand shifts the same by subtracting a constant value of 128 therefrom toobtain the digitized U and V chrominance signals, respectively. Theshifting unit 1 is optional if the input chrominance signals are thedigitized U and V chrominance signals instead of the digitized Cb and Crchrominance signals.

The first combining unit 2 receives the digitized U and V chrominancesignals and the digitized Y luminance signal from the shifting unit 1.The first combining unit 2 includes chrominance combination blocks 21,22, 23, 24, 25 for generating the linearly combined chrominance signals2V, 2U+V, 2(U+V), U+2V and 2U, respectively, and a Y multiple block 26which outputs the Y luminance signal.

The multiplexed multiplication unit 3 includes two multiplexed look-uptables 31, 32. The first multiplexed look-up table 31 receives thecombined chrominance signals from the chrominance combination blocks 21,22, 23 of the first combining unit 2, and contains digitizedtransformation values for performing the matrix multiplications whichinvolve the coefficient 0.714. The second multiplexed look-up table 32receives the combined chrominance signals from the chrominancecombination blocks 23, 24, 25 of the first combining unit 2, andcontains digitized transformation values for performing the matrixmultiplications which involve the coefficient 0.344.

The second combining unit 4 includes four adders 41, 42, 43, 44 whichreceive partial values outputted by the multiplexed look-up tables 31,32 and the Y luminance signal from the Y multiple block 26, and whichcombine the same to generate a set of RGB color combination signals,namely R-G, B-G and R+B-G.

The third combining unit 5 includes three subtracting units 51, 52, 53which receive the RGB color combination signals from the secondcombining unit 4 and which combine the same to generate the RGB colorspace signals, respectively.

FIG. 2 illustrates the multiplexed multiplication unit 3 of the firstpreferred embodiment in greater detail. As shown, the first multiplexedlook-up table 31 includes a first multiplexer 311 having three datainputs a1, b1, c1, a first look-up table 312 having an input connectedto a data output of the first multiplexer 311, and three data latches313, 314, 315, each of which is connected to an output of the firstlook-up table 312. The first multiplexer 311 selects each of the datainputs a1, b1, cl in three sequential timing phases t0, t1, t2, andprovides the selected data at the output thereof. The first look-uptable 312 outputs the digitized transformation value corresponding tothe product of the data at the input thereof and the coefficient 0.714.The latches 313, 314, 315 latch the outputs of the first look-up table312 during the sequential timing phases to, t1, t2, respectively.

The second multiplexed look-up table 32 includes a second multiplexer321 having three data inputs a2, b2, c2, a second look-up table 322having an input connected to a data output of the second multiplexer321, and three data latches 323, 324, 325, each of which is connected toan output of the second look-up table 322. The second multiplexer 321selects each of the data inputs a2, b2, c2 in three sequential timingphases t0, t1, t2, and provides the selected data at the output thereof.The second look-up table 322 outputs the digitized transformation valuecorresponding to the product of the data at the input thereof and thecoefficient 0.344. The latches 323, 324, 325 latch the outputs of thesecond look-up table 322 during the sequential timing phases t0, t1, t2,respectively. Preferably, the timing phases t0, t1, t2 for the first andsecond multiplexed look-up table units 31, 32 occur simultaneously.

Referring to FIG. 3, the second preferred embodiment of a color spaceconverting apparatus for converting digitized YCbCr color space signalsto digitized RGB color space signals in accordance with this inventionis shown to comprise a shifting unit 1, a first combining unit 7, amultiplexed multiplication unit 8, a second combining unit 9, and athird combining unit 10. In this embodiment, color space conversion isperformed according to the following equations:

    R-G=0.714(2V)+0.344(U+2V)                                  (c.1)

    B-G=0.714(2U+V)+0.344(2U)                                  (c.2)

    B+G=2Y+0.714(2U-V)                                         (c.4)

It is noted that Equations (c.1), (c.2) and (c.4) involve threemultiplication operations using the coefficient 0.714, and twomultiplication operations using the coefficient 0.344.

The first combining unit 7 receives the digitized U and V chrominancesignals and the digitized Y luminance signal from the shifting unit 1.The first combining unit 7 includes chrominance combination blocks 71,72, 73, 74, 75 for generating the combined chrominance signals 2V, 2U+V,2U-V, U+2V and 2U, respectively, and a Y multiple block 76 forgenerating the 2Y luminance signal.

The multiplexed multiplication unit 8 includes two multiplexed look-uptables 81, 82. The first multiplexed look-up table 81 receives thecombined chrominance signals from the chrominance combination blocks 71,72, 73 of the first combining unit 7, and contains digitizedtransformation values for performing the matrix multiplications whichinvolve the coefficient 0.714. The second multiplexed look-up table 82receives the combined chrominance signals from the chrominancecombination blocks 74, 75 of the first combining unit 7, and containsdigitized transformation values for performing the matrixmultiplications which involve the coefficient 0.344.

The second combining unit 9 includes three adders 91, 92, 93 whichreceive partial values outputted by the multiplexed look-up tables 81,82 and the 2Y luminance signal from the Y multiple block 76, and whichcombine the same to generate a set of RGB color combination signals,namely R-G, B-G and B+G.

The third combining unit 10 includes an adder 101 for adding the B-G andB+G color combination signals, a shifter 102 for shifting the output ofthe adder 101 to obtain the B color space signal, a subtracting unit 103for generating the difference of the B-G and B+G color combinationsignals, a shifter 104 for shifting the output of the subtracting unit103 to obtain the G color space signal, and an adder 105 for adding theR-G color combination signal and the G color space signal to obtain theR color space signal.

The multiplexed multiplication unit 8 is generally similar to that shownin FIG. 2, except that the second multiplexed look-up table 82 includesa second multiplexer (not shown) having two data inputs that areselected in two sequential timing phases, a second look-up table (notshown), and two latches (not shown) which latch the output of the secondlook-up table during the sequential timing phases, respectively.

Referring to FIG. 4, the third preferred embodiment of a color spaceconverting apparatus for converting digitized YCbCr color space signalsto digitized RGB color space signals in accordance with this inventionis shown to comprise a shifting unit 1, a first combining unit 12, amultiplexed multiplication unit 13, a second combining unit 14, and athird combining unit 15. In this embodiment, color space conversion isperformed according to the following equations:

    R-G=0.714(2V)+0.344(U+2V)                                  (c.1)

    B+G=2Y+0.714(2U-V)                                         (c.4)

    R+G=2Y+0.344(2V-U)                                         (c.5)

It is noted that Equations (c.1), (c.4) and (c.5) involve twomultiplication operations using the coefficient 0.714, and twomultiplication operations using the coefficient 0.344.

The first combining unit 12 receives the digitized U and V chrominancesignals and the digitized Y luminance signal from the shifting unit 1.The first combining unit 12 includes chrominance combination blocks 121,122, 123, 124 for generating the combined chrominance signals 2V, 2U-V,U+2V and 2V-U, respectively, and a Y multiple block 125 for generatingthe 2Y luminance signal.

The multiplexed multiplication unit 13 includes two multiplexed look-uptables 131, 132. The first multiplexed look-up table 131 receives thecombined chrominance signals from the chrominance combination blocks121, 122 of the first combining unit 12, and contains digitizedtransformation values for performing the matrix multiplications whichinvolve the coefficient 0.714. The second multiplexed look-up table 132receives the combined chrominance signals from the chrominancecombination blocks 123, 124 of the first combining unit 12, and containsdigitized transformation values for performing the matrixmultiplications which involve the coefficient 0.344.

The second combining unit 14 includes three adders 141, 142, 143 whichreceive partial values outputted by the multiplexed look-up tables 131,132 and the 2Y luminance signal from the Y multiple block 125, and whichcombine the same to generate a set of RGB color combination signals,namely R-G, R+G and B+G.

The third combining unit 15 includes an adder 151 for adding the R-G andR+G color combination signals, a shifter 152 for shifting the output ofthe adder 151 to obtain the R color space signal, a subtracting unit 153for generating the difference of the R-G and R+G color combinationsignals, a shifter 154 for shifting the output of the subtracting unit153 to obtain the G color space signal, and a subtracting unit 155 forgenerating the difference of the B+G color combination signal and the Gcolor space signal to obtain the B color space signal.

The multiplexed multiplication unit 13 is generally similar to thatshown in FIG. 2, except that each of the multiplexed look-up tables 131,132 includes a multiplexer (not shown) having two data inputs that areselected in two sequential timing phases, a look-up table (not shown),and two latches (not shown) which latch the output of the look-up tableduring the sequential timing phases, respectively.

It is noted that the preceding embodiments can be modified by changingthe binary coefficients of the combined chrominance signals and thebinary combination of the luminance signal from the first combiningunit, and by modifying the subsequent processing units for compensationpurposes. In the embodiments of FIGS. 5 to 8, the combined chrominancesignals from the chrominance combination blocks 126, 127, 128, 129 ofthe first combining unit 12a, 12b are half of those from the chrominancecombination blocks 121, 122, 123, 124 of the first combining unit 12 ofthe embodiment of FIG. 4.

Referring to FIG. 5, in order to ensure proper operation of the colorspace converting apparatus based on the same set of conversion formulasused in the embodiment of FIG. 4, the multiplexed multiplication unit13a of this embodiment includes two multiplexed look-up tables 133, 134,the values contained therein being twice those found in the multiplexedlook-up tables 131, 132 of the multiplexed multiplication unit 13 so asto compensate for differences in the combined chrominance signals fromthe first combining units 12, 12a without modifying the second and thirdcombining units 14, 15.

Referring to FIG. 6, in order to ensure proper operation of the colorspace converting apparatus based on the same set of conversion formulasused in the embodiment of FIG. 4 without modifying the multiplexedmultiplication unit 13 and the third combining unit 15, the secondcombining unit 14a of this embodiment further includes four shifters144, 145, 146, 147 which interconnect the multiplexed multiplicationunit 13 and the adders 141, 142, 143. The shifters 144, 145, 146, 147perform a left shift of the outputs of the multiplexed look-up tables131, 132 to compensate for the division of the combined chrominancesignals from the first combining unit 12a.

In the embodiment of FIG. 7, the Y multiple block 130 of the firstcombining unit 12b outputs the Y luminance signal instead of the 2Yluminance signal as generated by the Y multiple block 125 of the firstcombining unit 12a of the embodiments of FIGS. 5 and 6. In order toensure proper operation of the color space converting apparatus based onthe same set of conversion formulas used in the embodiment of FIG. 4without modifying the multiplexed multiplication unit 13 and the thirdcombining unit 15, the second combining unit 14b of this embodimentfurther includes three shifters 148, 149, 150 which interconnect theadders 141, 142, 143 and the third combining unit 15. The shifters 148,149, 150 perform a left shift of the outputs of the adders 141, 142, 143to compensate for the division of the combined chrominance signals fromthe first combining unit 12b.

FIG. 8 illustrates still another embodiment of the present invention. Asshown, using the first combining unit 12b of FIG. 7 and the multiplexedmultiplication unit 13 and the second combining unit 14 of FIG. 4, thethird combining unit 15a is modified in order to ensure proper operationof the color space converting apparatus based on the same set ofconversion formulas used in the embodiment of FIG. 4. As shown, thethird combining unit 15a includes an adder 156 for adding the (R-G)/2and (R+G)/2 color combination signals from the adders 141, 142 of thesecond combining unit 14 to obtain the R color space signal, asubtracting unit 157 for generating the difference of the (R-G)/2 and(R+G)/2 color combination signals to obtain the G color space signal, ashifter 158 for performing a left shift of the (B+G)/2 color combinationsignal from the adder 143 of the second combining unit 14, and asubtracting unit 159 for generating the difference of the B+G colorcombination signal from the shifter 158 and the G color space signalfrom the subtracting unit 157 to obtain the B color space signal.

Note that, aside from the conversion formulas of Equations (c.1) to(c.5), Equations (b.1) to (b.3) can be further rearranged as follows:

    B-G=0.714 (3-δ)U+V!                                  (d.1)

    R+B-G=Y+0.714 (3-δ)(U+V)!                            (d.2)

    B+G=2Y+0.714(2U-V)                                         (d.3)

    R-G=0.344 (6+ε)V+U!                                (d.4)

    R+B-G=Y+0.344 (6+ε)(V+U)                           (d.5)

    R+G=2Y+0.344(2V-U)                                         (d.6)

where δ=0.036, ε=0.151.

Equations (d.1) to (d.6) mean that, by tolerating an error term, the useof only one look-up table is permitted in the conversion of digitizedluminance-chrominance color space signals to digitized RGB color spacesignals.

Referring to FIG. 9, the eighth preferred embodiment of a color spaceconverting apparatus for converting digitized YCbCr color space signalsto digitized RGB color space signals in accordance with this inventionis shown to comprise a shifting unit 1, a first combining unit 17, amultiplexed multiplication unit 18, a second combining unit 19, and athird combining unit 20. In this embodiment, color space conversion isperformed according to the following equations:

    B-G=0.714 (3-δ)U+V!                                  (d.1)

    R+B-G=Y+0.714 (3-δ)(U+V)                             (d.2)

    B+G=2Y+0.714(2U-V)                                         (d.3)

It is noted that Equations (d.1), (d.2) and (d.3) only involve threemultiplication operations using the coefficient 0.714 if the error termδ is to be ignored.

The first combining unit 17 receives the digitized U and V chrominancesignals and the digitized Y luminance signal from the shifting unit 1.The first combining unit 17 includes chrominance combination blocks 171,172, 173 for generating the combined chrominance signals 3U+V, 2U-V and3(U+V), respectively, and two Y multiple blocks 174, 175 for outputtingthe 2Y and Y luminance signals, respectively.

The multiplexed multiplication unit 18 includes a multiplexer (notshown) having three data inputs that receive the combined chrominancesignals from the chrominance combination blocks 171, 172, 173 of thefirst combining unit 17 and that are selected in three sequential timingphases, a look-up table (not shown) containing digitized transformationvalues for performing the matrix multiplications which involve thecoefficient 0.714, and three latches (not shown) which latch the outputof the look-up table during the sequential timing phases, respectively.

The second combining unit 19 includes two adders 191, 192 which receivepartial values outputted by the multiplexed multiplication unit 18 andthe combined luminance signals from the Y multiple blocks 174, 175 andwhich combine the same to generate a set of R'G'B' color combinationsignals, namely B'+G', B'-G' and R'+B'-G'.

The third combining unit 20 includes a subtracting unit 205 forgenerating the difference of the R'+B'-G' and B'-G' color combinationsignals to obtain the R' color signal, a subtracting unit 203 forgenerating the difference of the B'+G' and B'-G' color combinationsignals, a shifter 204 for shifting the output of the subtracting unit203 to obtain the G' color signal, an adder 201 for adding the B'+G' andB'-G' color combination signals, and a shifter 202 for shifting theoutput of the adder 201 to obtain the B' color signal. The R', G' and B'color signals approximate the R, G and B color space signalsrespectively by a corresponding small error code eR, eG, eB because theerror term δ in Equations (d.1) to (d.3) was ignored. The error codeseR, eG, eB can be calculated as follows:

eB=1/2(-0.714*δ*U); eB ranges between -1.65, 1.66!

eG=1/2(0.714*δ*U); eG ranges between -1.66, 1.65!

eR=-0.714*δ*V; eR ranges between -3.30, 3.33!

The calculated errors are insignificant in terms of color because theirlevels are relatively small, especially for applications that do notrequire further conversion of the resultant RGB color space signals.

Similarly, only one coefficient 0.344 is needed if color spaceconversion is performed according to the Equations (d.4), (d.5) and(d.6) mentioned beforehand. The error codes eB, eG, eR rangerespectively between -3.32, 3.301, -3.30, 3.32! and -6.65, 6.60!if theerror term ε was ignored.

Referring to FIG. 10, the embodiment of FIG. 9 can be modified so as tocompensate the R'G'B' color signals to obtain the precise RGB colorspace signals. As shown, a first error code generator 11 receives thedigitized V chrominance signal and generates the error compensating codeeR for the R color space signal in accordance with Table I.

                  TABLE I    ______________________________________    V                  eR    ______________________________________    {97, 98, . . . 127}                       3    {58, 59, . . . 96} 2    {20, 21, . . . 57} 1    {-19, -18, . . . 19}                       0    {-57, -56, . . . -20}                       -1    {-96, -95, . . . -58}                       -2    {-128, -127, . . . -97}                       -3    ______________________________________

A second error code generator 16 receives the digitized U chrominancesignal and generates the error compensating codes eG, eB for the G and Bcolor space signals in accordance with Table II.

                  TABLE II    ______________________________________    U                 eG, eB    ______________________________________    {116, 117, . . . 127}                      2    {39, 40, . . . 115}                      1    {-38, -37, . . . 38}                      0    {-115, -114, . . . -39}                      -1    {-128, -127, . . . -116}                      -2    ______________________________________

An adder 110 adds the error compensating code eR to the R' color signalto obtain the R color space signal. An adder 161 adds the errorcompensating code eG to the G' color signal to obtain the G color spacesignal. An adder 162 adds the error compensating code eB to the B' colorsignal to obtain the B color space signal. Note that the errorcompensating codes eB and eG are identical. In addition, all errorcompensating codes eR, eG, eB are rounded to the nearest integer.

Although the preferred embodiments are shown as hard-wired blocks in adedicated hardware circuitry, the present invention may be easily andefficiently implemented using a microprocessor and the associated systemmemory to implement the calculation of the conversion.

It has thus been shown that, in the present invention,luminance-chrominance to RGB color space conversion can be implementedusing no more than two look-up tables. Thus, a relatively inexpensiveand highly efficient color space converting method and apparatus hasbeen realized. The object of the present invention is thus met.

While the present invention has been described in connection with whatis considered the most practical and preferred embodiments, it isunderstood that this invention is not limited to the disclosedembodiments but is intended to cover various arrangements includedwithin the spirit and scope of the broadest interpretation so as toencompass all such modifications and equivalent arrangements.

I claim:
 1. A method for converting digitized luminance-chrominancecolor space signals to digitized RGB color space signals, comprising thesteps of:generating a plurality of predetermined linear combinations ofthe chrominance color space signals and at least one predeterminedbinary combination of the luminance color space signal; providing amultiplexed multiplication unit which receives the linear combinationsof the chrominance color space signals, the multiplexed multiplicationunit including no more than two look-up tables which contain digitizedtransformation values for performing matrix multiplications of thelinear combinations of the chrominance color space signals; linearlycombining the digitized transformation values outputted by themultiplexed multiplication unit and the binary combination of theluminance color space signal in a first predetermined manner to generatethree RGB color combination signals; and linearly combining the RGBcolor combination signals in a second predetermined manner to obtain theRGB color space signals.
 2. The method as claimed in claim 1, whereinthe chrominance color space signals are U and V color space signals. 3.The method as claimed in claim 1, wherein the chrominance color spacesignals are Cb and Cr color space signals, the method further comprisingthe step of shifting each of the Cb and Cr color space signals by apredetermined constant to obtain U and V color space signals,respectively, before generating the linear combinations of the U and Vcolor space signals.
 4. The method as claimed in claim 1, wherein themultiplexed multiplication unit comprises:a first multiplexed look-uptable including: a first multiplexer having a plurality of data inputswhich receive selected ones of the linear combinations of thechrominance color space signals, and a data output, the firstmultiplexer selecting each of the data inputs thereof in sequentialfirst timing phases and providing data present at the selected one ofthe data inputs to the data output thereof; a first one of the look-uptables having an input connected to the data output of the firstmultiplexer, the first one of the look-up tables outputting thedigitized transformation value corresponding to product of the data atthe input thereof and a predetermined first coefficient; and a pluralityof first data latches, each of which is connected to the first one ofthe look-up tables so as to latch outputs of the first one of thelook-up tables during the sequential first timing phases, respectively;and a second multiplexed look-up table including: a second multiplexerhaving a plurality of data inputs which receive selected ones of thelinear combinations of the chrominance color space signals, and a dataoutput, the second multiplexer selecting each of the data inputs thereofin sequential second timing phases and providing data present at theselected one of the data inputs to the data output thereof; a second oneof the look-up tables having an input connected to the data output ofthe second multiplexer, the second one of the look-up tables outputtingthe digitized transformation value corresponding to product of the dataat the input thereof and a predetermined second coefficient; and aplurality of second data latches, each of which is connected to thesecond one of the look-up tables so as to latch outputs of the secondone of the look-up tables during the sequential second timing phases,respectively.
 5. The method as claimed in claim 4, wherein the first andsecond timing phases occur simultaneously.
 6. The method as claimed inclaim 1, wherein the multiplexed multiplication unit includes no morethan one look-up table and comprises a multiplexed look-up tableincluding: a multiplexer having a plurality of data inputs which receivethe linear combinations of the chrominance color space signals, and adata output, the multiplexer selecting each of the data inputs thereofin sequential timing phases and providing data present at the selectedone of the data inputs to the data output thereof; said one look-uptable having an input connected to the data output of the multiplexer,said one look-up table outputting the digitized transformation valuecorresponding to product of the data at the input thereof and apredetermined coefficient; and a plurality of data latches, each ofwhich is connected to said one look-up table so as to latch outputs ofsaid one look-up table during the sequential timing phases,respectively.
 7. The method as claimed in claim 6, further comprisingthe step of adding error compensating codes to the RGB color spacesignals.
 8. An apparatus for converting digitized luminance-chrominancecolor space signals to digitized RGB color space signals, said apparatuscomprising:a first combining unit for generating a plurality ofpredetermined linear combinations of the chrominance color space signalsand at least one predetermined binary combination of the luminance colorspace signal; a multiplexed multiplication unit connected to said firstcombining unit to receive the linear combinations of the chrominancecolor space signals therefrom, said multiplexed multiplication unitincluding no more than two look-up tables which contain digitizedtransformation values for performing matrix multiplications of thelinear combinations of the chrominance color space signals; a secondcombining unit connected to said multiplexed multiplication unit andsaid first combining unit, said second combining unit linearly combiningthe digitized transformation values outputted by said multiplexedmultiplication unit and the binary combination of the luminance colorspace signal in a first predetermined manner to generate three RGB colorcombination signals; and a third combining unit connected to said secondcombining unit, said third combining unit linearly combining the RGBcolor combination signals in a second predetermined manner to obtain theRGB color space signals.
 9. The apparatus as claimed in claim 8, whereinthe chrominance color space signals are U and V color space signals. 10.The apparatus as claimed in claim 8, wherein the chrominance color spacesignals are Cb and Cr color space signals, said apparatus furthercomprising a shifting unit connected to said first combining unit, saidshifting unit shifting each of the Cb and Cr color space signals by apredetermined constant to obtain U and V color space signals,respectively, that are provided to said first combining unit.
 11. Theapparatus as claimed in claim 10, wherein said shifting unit subtractsthe predetermined constant from each of the Cb and Cr color spacesignals.
 12. The apparatus as claimed in claim 8, wherein saidmultiplexed multiplication unit comprises:a first multiplexed look-uptable including: a first multiplexer having a plurality of data inputswhich receive selected ones of the linear combinations of thechrominance color space signals, and a data output, said firstmultiplexer selecting each of said data inputs thereof in sequentialfirst timing phases and providing data present at the selected one ofsaid data inputs to said data output thereof; a first one of saidlook-up tables having an input connected to said data output of saidfirst multiplexer, said first one of said look-up tables outputting thedigitized transformation value corresponding to product of the data atsaid input thereof and a predetermined first coefficient; and aplurality of first data latches, each of which is connected to saidfirst one of said look-up tables so as to latch outputs of said firstone of said look-up tables during the sequential first timing phases,respectively; and a second multiplexed look-up table including: a secondmultiplexer having a plurality of data inputs which receive selectedones of the linear combinations of the chrominance color space signals,and a data output, said second multiplexer selecting each of said datainputs thereof in sequential second timing phases and providing datapresent at the selected one of said data inputs to said data outputthereof; a second one of said look-up tables having an input connectedto said data output of said second multiplexer, said second one of saidlook-up tables outputting the digitized transformation valuecorresponding to product of the data at said input thereof and apredetermined second coefficient; and a plurality of second datalatches, each of which is connected to said second one of said look-uptables so as to latch outputs of said second one of said look-up tablesduring the sequential second timing phases, respectively.
 13. Theapparatus as claimed in claim 12, wherein the first and second timingphases occur simultaneously.
 14. The apparatus as claimed in claim 8,wherein said multiplexed multiplication unit includes no more than onelook-up table and comprises a multiplexed look-up table including: amultiplexer having a plurality of data inputs which receive the linearcombinations of the chrominance color space signals, and a data output,said multiplexer selecting each of said data inputs thereof insequential timing phases and providing data present at the selected oneof said data inputs to said data output thereof; said one look-up tablehaving an input connected to said data output of said multiplexer, saidone look-up table outputting the digitized transformation valuecorresponding to product of the data at said input thereof and apredetermined coefficient; and a plurality of data latches, each ofwhich is connected to said one look-up table so as to latch outputs ofsaid one look-up table during the sequential timing phases,respectively.
 15. The apparatus as claimed in claim 14, furthercomprising a compensating unit connected to said third combining unit,said compensating unit adding error compensating codes to the RGB colorspace signals.